The nuclear industry annually produces a significant amount of waste which is classified as radioactively contaminated ion exchange media. In Sweden, such waste is managed in various fashions in the individual nuclear facilities prior to ultimate disposal in bedrock chambers. This management is technically complex and as a rule leads to increased volumes which influences storage costs. A process resulting in diminished volume at reasonable cost should therefore be commercially interesting.
Ion exchange medium is an organic material. The base is usually a styrene polymer with grafted sulphonic acid and amine groups. The material is therefore burnable, but air is supplied during combustion and sulphur and nitrogen oxides are formed which in turn must be separated in some manner. Additionally, during combustion the temperature becomes sufficiently high for radioactive caesium to be partially vapourised. The residual radioactivity will also accompany the resulting fly ash to some extent. This necessitates a very high performance filter system. Accordingly, both technical and economic problems are associated with the combustion technique.
An alternative to combustion is pyrolysis. However, previously known pyrolysis methods in this technical field are deficient in several aspects and in particular no one has earlier succeeded in devising a pyrolysis process which provides a comprehensive solution to the problem of sulphur and nitrogen-containing radioactive waste, and to do so under acceptable economic stipulations. The following can be mentioned as examples of the known technology in this respect:
SE-B 8405113-5 which describes single stage pyrolysis in a fluidised bed followed by conversion of tars in the resulting gas to non-condensable gas using limestone as catalyst.
U.S. Pat. No. 4,628,837, U.S. Pat. No. 4,636,335 and U.S. Pat. No. 4,654,172 which all describe pyrolysis of ion exchange resins where the pyrolysis is certainly carried out in two stages but where both of these stages are directed towards pyrolysis of the ion exchange media itself i.e. the solid product. Speaking generally, both stages moreover are carried out at relatively low temperatures. Furthermore, none of these specifications recites any comprehensive solution to the problem of solid organic sulphur-containing waste such as is the case with the method of the present invention.